1. Field of the Invention
This invention relates to the dense packaging of electronic circuitry, and specifically to the stacking of integrated circuit (IC) chips, or die.
2. Description of the Prior Art
Stackable IC chip layers were disclosed in U.S. Pat. No. 5,953,588 which permit chips having different functions and therefore different areas to be stacked as if they were same size chips, using stacking and electrical connection techniques and tools which have been developed for same size chips. The new units were referred to as “neo” (or “pseudo” or “virtual”) chips. In addition to the advantage of being able to use chips of varying sizes in a given stack, that technology permits the processing and stacking of chips purchased as individual die, which are more readily available than chips purchased in wafer form. Furthermore, the chips purchased as individual die are generally “known good” die, which have been “burned in”, and are therefore pre-tested prior to stacking.
A re-wafering process is used, in which a neo-wafer is formed encapsulating known good chips, so that the chips can be prepared for stacking by covering their active surfaces with a dielectric layer, forming vias through the dielectric layer to reach the terminals on the respective chips, and metallizing to provide electrical connections from the chip terminals to side surfaces of the layer, which are created when the neo-wafer is diced, or sliced, to provide individual layers ready for stacking.
Prior to this technology the extra steps required preparatory to stacking were sometimes carried out while the chips were still in their original wafer form. The wafer concept is almost universally used to simultaneously form integrated circuits (ICs) in numerous locations in the wafer, so that a multiplicity of separate IC chips will be created when the wafer is diced. Since preparation for stacking requires that the chip surfaces be metalized to connect their terminals to suitable access planes on the stack, manufacturing steps beyond the normal wafer processing steps are required, if stacking is intended. In some cases, chips in TSOP (packaged) form have been electrically connected to external circuitry by means of metal frames which are formed as part of the TSOP structure.
In this process the chips which have been previously formed in a wafering process, and tested to insure their performance, need to be re-wafered, so they can be processed for subsequent stacking. Even in the case of a single chip, it is not feasible to perform the pre-stacking processing steps without using a neo-wafer, which proves a large enough body to permit efficient handling. Of course, the manufacturing process is much more cost effective if the neo-wafer contains a plurality of pre-formed, pre-tested chips which can be simultaneously prepared for stacking. The neo-wafer is subsequently diced to form individual layers ready for stacking. In effect, two wafering and dicing processes are used to facilitate stacking of chip-encapsulating layers.
The primary challenge in using a neo-wafer containing multiple die is the accurate location of each die. With multiple die in the wafer, the accuracy necessary to locate each die prior to potting creates a potential alignment problem.
The re-wafering process was improved in U.S. Pat. No. 6,117,704 by making location of chips (die) in the neo-wafer highly accurate, and by making it more feasible to include multiple chips in the neo-wafer. The neo-wafer is so structured that each chip it contains is precisely located by use of a single masking step to obtain exact location of the known good chips, which are inserted in the neo-wafer and then covered by potting material. Then the chips in the neo-wafer are simultaneously processed to prepare them for stacking. They are stacked after they have been diced from the neo-wafer.
Since, with the present invention, the locations of the chips in the neo-wafer are controlled by photo patterning (photo-lithography), their relative locations are determined with the very high precision inherent in the use of a single mask to control the locations of all chips in the neo-wafer.
Another advantage of the improved process is that any leakage of potting material onto the active (upper) surfaces of the chips in the neo-wafer does not affect the electrical terminals on the chips, so that removal of any such leaked material is not required, and increased reliability of the electrical connection is obtained.
The benefits of the improved prior art are obtained by using a wafer frame, sometimes called a “picture frame”, having a plurality of separate chambers, or cavities, into which the individual known good chips are inserted. The chips have conductive bumps which extend into and through pre-formed small holes (vias) in the surface of the wafer frame. The via holes have been formed using photo-lithography with a single mask for the entire neo-wafer, thus insuring precise location of the via holes relative to one another. This insures precise location of the separate chips relative to one another, and provides terminals accessible for electrical connections.